The present invention relates to an MR imaging method, residual magnetization amount measuring method and MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method capable of restraining residual magnetization caused by a previous pulse sequence from affecting a current pulse sequence, a residual magnetization amount measuring method for verifying a demagnetizing effect, and an MRI apparatus for implementing such methods.
In the publications of Japanese Patent Application Laid Open Nos. H8-322817 and H10-75940, MR imaging methods are disclosed in which a corrective component is appended to a phase encoder gradient or rewind gradient for restraining residual magnetization caused by a phase encoder gradient from affecting an echo next to an echo that corresponds to the phase encoder gradient.
In MR imaging, a pulse sequence involving transmitting an RF pulse, applying a phase encoder gradient and receiving an NMR signal from a subject is repeated with the phase encoding amount varied to thereby collect data for filling the k-space.
The aforementioned conventional techniques restrain the effect of residual magnetization within one pulse sequence, and do not restrain residual magnetization caused by a previous pulse sequence from affecting the current pulse sequence.
It is therefore an object of the present invention to provide an MR imaging method capable of restraining residual magnetization caused by a previous pulse sequence from affecting the current pulse sequence, a residual magnetization amount measuring method for verifying a demagnetizing effect, and an MRI apparatus for implementing such methods.
In its first aspect, the present invention provides an MR imaging method for transmitting an RF pulse, applying a phase encoder gradient, receiving an NMR signal from a subject and producing an MR image based on said NMR signal, characterized in: prior to transmitting the RF pulse, consecutively applying first through I-th (xe2x89xa72) gradient pulses having alternately inverting polarity and pulse heights reduced in order.
In accordance with the MR imaging method of the first aspect, since first through I-th gradient pulses having alternately inverting polarity and pulse heights reduced in order are consecutively applied prior to substantially beginning a pulse sequence for acquiring data, residual magnetization of a previous pulse sequence is demagnetized, and thereafter the pulse sequence for acquiring data is substantially begun. Therefore, residual magnetization caused by the previous pulse sequence can be restrained from affecting the current pulse sequence.
In its second aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that said first gradient pulse has a pulse height such as to saturate residual magnetization.
In accordance with the MR imaging method of the second aspect, since a first gradient pulse having a pulse height such as to saturate residual magnetization is applied, even if residual magnetization remains after applying the I-th gradient pulse, the amount of the residual magnetization is always constant. For example, in a pulse sequence according to a spin echo technique, while a varying amount of residual magnetization causes artifacts on an MR image, an always constant amount of residual magnetization does not generate artifacts on an MR image. On the other hand, in a pulse sequence according to a fast spin echo technique, even a constant amount of residual magnetization causes artifacts on an MR image as it stands because an offset in an odd-numbered echo and an offset in an even-numbered echo are in opposite directions; however, the offset amounts are constant and therefore it is possible to correct data. Thus, residual magnetization caused by the previous pulse sequence can be restrained from affecting the current pulse sequence.
In its third aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that I=4.
If I is increased, residual magnetization can be further reduced but this processing is time consuming. On the other hand, if I is decreased, the processing time can be reduced but the effect of decreasing residual magnetization is also reduced.
Therefore, in accordance with the MR imaging method of the third aspect, I=4. Thus, tradeoff between the effect of decreasing residual magnetization and the processing time can be optimized.
In its fourth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the first through I-th gradient pulses have trapezoidal pulse waveforms.
In accordance with the MR imaging method of the fourth aspect, by shaping the pulse waveforms as trapezoids, they can be raised at the maximum slew rate possible with the hardware.
In its fifth aspect, the present invention provides a residual magnetization amount measuring method characterized in: consecutively applying first through I-th (xe2x89xa72) gradient pulses having alternately inverting polarity and pulse heights reduced in order; applying a 90xc2x0 RF pulse; applying a 180xc2x0 RF pulse; observing a first echo while applying a read gradient; transmitting a 180xc2x0 RF pulse; observing a second echo while applying a read gradient; and measuring a residual magnetization amount from offsets of echo peaks of said first and second echoes.
The effect of residual magnetization after applying the gradient pulses appears as an offset of an echo peak from an echo center. The offset of an echo peak from an echo center is, however, difficult to measure because the echo center is difficult to identify when a single echo is taken.
Therefore, in accordance with the residual magnetization amount measuring method of the fifth aspect, an echo peak of a first echo and that of a second echo are compared. Since an offset of the echo peak from the echo center of the first echo and an offset of the echo peak from the echo center of the second echo are in opposite directions, half of the offsets of the echo peaks of the first and second echoes gives the offset of an echo peak from an echo center. Hence, the residual magnetization amount can be determined from the offset. That is, the effect of applying the gradient pulses can be evaluated.
In its sixth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in adjusting at least one of the pulse height and the pulse width of the second through I-th gradient pulses based on the residual magnetization amount measured by the residual magnetization amount measuring method of the aforementioned configuration.
In accordance with the MR imaging method of the sixth aspect, the pulse width and the pulse height of the gradient pulses can be optimized.
In its seventh aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the pulse height of an i-th gradient pulse is half the pulse height of an (ixe2x88x921)-th gradient pulse.
In accordance with the MR imaging method of the seventh aspect, since the pulse heights of the gradient pulses are halved in order, the processing can be simplified.
In its eighth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the pulse widths of the first through I-th gradient pulses are substantially the same.
In accordance with the MR imaging method of the eighth aspect, since the pulse width is not varied among the gradient pulses, the processing can be simplified.
In its ninth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in applying the first through I-th gradient pulses to a gradient axis to which a phase encoder gradient is applied.
It is the residual magnetization caused by a phase encoder gradient that especially becomes an issue in MR imaging.
Therefore, in accordance with the MR imaging method of the ninth aspect, the effect of residual magnetization caused by a phase encoder gradient is particularly reduced by applying the aforementioned gradient pulses to a gradient axis to which the phase encoder gradient is applied. In some pulse sequences having a plurality of gradient axes to which the phase encoder gradient is applied, the above gradient pulses are applied to those gradient axes.
In its tenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means and NMR signal receiving means, said apparatus controlling said means to transmit an RF pulse, apply a phase encoder gradient, receive an NMR signal from a subject and produce an MR image based on said NMR signal, characterized in comprising: residual magnetization demagnetizing means for, prior to transmitting the RF pulse, consecutively applying first through I-th (xe2x89xa72) gradient pulses having alternately inverting polarity and pulse heights reduced in order.
In accordance with the MRI apparatus of the tenth aspect, the MR imaging method of the first aspect can be suitably implemented.
In its eleventh aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that said first gradient pulse has a pulse height such as to saturate residual magnetization.
In accordance with the MRI apparatus of the eleventh aspect, the MR imaging method of the second aspect can be suitably implemented.
In its twelfth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that I=4.
In accordance with the MRI apparatus of the twelfth aspect, the MR imaging method of the third aspect can be suitably implemented.
In its thirteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the first through I-th gradient pulses have trapezoidal pulse waveforms.
In accordance with the MRI apparatus of the thirteenth aspect, the MR imaging method of the fourth aspect can be suitably implemented.
In its fourteenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means and NMR signal receiving means, said apparatus controlling said means to transmit an RF pulse, apply a phase encoder gradient, receive an NMR signal from a subject and produce an MR image based on said NMR signal, characterized in comprising: residual magnetization amount measuring means for consecutively applying first through I-th (xe2x89xa72) gradient pulses having alternately inverting polarity and pulse heights reduced in order; applying a 90xc2x0 RF pulse; applying a 180xc2x0 RF pulse; observing a first echo while applying a read gradient; transmitting a 180xc2x0 RF pulse; observing a second echo while applying a read gradient; and measuring a residual magnetization amount from offsets of echo peaks of said first and second echoes.
In accordance with the MRI apparatus of the fourteenth aspect, the MR imaging method of the fifth aspect can be suitably implemented.
In its fifteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in adjusting at least one of the pulse height and the pulse width of the second through I-th gradient pulses based on the residual magnetization amount measured by the residual magnetization amount measuring means of the aforementioned configuration.
In accordance with the MRI apparatus of the fifteenth aspect, the MR imaging method of the sixth aspect can be suitably implemented.
In its sixteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the pulse height of an i-th gradient pulse is half the pulse height of an (ixe2x88x921)-th gradient pulse.
In accordance with the MRI apparatus of the sixteenth aspect, the MR imaging method of the seventh aspect can be suitably implemented.
In its seventeenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the pulse widths of the first through I-th gradient pulses are substantially the same.
In accordance with the MRI apparatus of the seventeenth aspect, the MR imaging method of the eighth aspect can be suitably implemented.
In its eighteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in applying the first through I-th gradient pulses to a gradient axis to which a phase encoder gradient is applied.
In accordance with the MRI apparatus of the eighteenth aspect, the MR imaging method of the ninth aspect can be suitably implemented.
According to the MR imaging method and MRI apparatus of the present invention, a demagnetizing gradient pulse sequence is applied prior to an MR imaging pulse sequence to cancel residual magnetization caused by a previous MR imaging pulse sequence and reduce the residual magnetization. Thus, the residual magnetization caused by the previous MR imaging pulse sequence can be restrained from affecting an MR image, and the image quality of MR images can be improved.
Moreover, according to the residual magnetization amount measuring method and MRI apparatus of the present invention, the residual magnetization amount caused by the demagnetizing gradient pulse sequence can be measured. Thus, the demagnetizing effect of the demagnetizing gradient pulse sequence can be evaluated.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.